Tene suggested I put this here. I was looking through bukkit plugins that might be handy learning and stumbled across a really cool plugin called JoinDate. It's a really simple plugin that with one command tells you the date you joined the server. It even works with players who joined before it was added.
I just think it'd be a nice little thing that people would enjoy having, especially older players who'd like to see how long they've been on the server. It could even be a donator perk, much like /hat. Also, it'd be useful for mod applications where they ask for how long you've been on the server.
It's only one command and, in turn, one permission. It really would not take up any space or cause lag and is updated and not out of date.
Link:
http://dev.bukkit.org/bukkit-plugins/joindate/